The referenced U.S. Pat. No. 3,612,664 describes a method and an apparatus to separate a crystallizable component from a liquid by fractional crystallization. The method now is industrially known as the Sulzer-MWB method, and, essentially, provides that a liquid is repeatedly fed through a crystallization apparatus. A crystal layer will separate out. After the crystal layer has reached a predetermined thickness, the crystal layer is melted. The apparatus utilizes a vertical pipe assembly. The liquid is fed through the pipe. A heat transfer medium is applied to the outside of the pipes to cool the pipes during crystallization or, selectively, to heat the pipes during melting.
It has been found, in operation, that as the crystallized layer increases in thickness on the tube walls, heat transferred to the tube walls is decreased. As the layer builds up, the efficiency of separation by crystallization decreases. Additionally, some materials cause problems in starting crystallization; at times, the formation of crystals which is desired is not obtained.